1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display substrate having thin film transistors or the like as switching elements, a liquid crystal display device and a fabrication method thereof.
2. Description of Related Art
The active matrix type liquid crystal display device is provided with nonlinear elements (or switching elements) which correspond individually to a plurality of pixel electrodes arrayed in a matrix shape. Since the liquid crystals in the individual pixels are theoretically driven at all times (or a duty ratio of 1.0), the active type has a higher contrast than that of the so-called "simple matrix type" adopting the time sharing drive type, and is considered to be indispensable for the color liquid crystal display device. The switching elements are represented by thin film transistors (TFT).
Here, the active matrix type liquid crystal display device using the thin film transistors is disclosed, for example, in Japanese Patent Application Laid-Open No. 309921/1988 or in "12.5 Type Active Matrix Color Liquid Crystal Display" on pp. 193 to 210 of Nikkei Electronics issued on Dec. 15, 1986 by Nikkei McGraw-Hill.
The liquid crystal display panel (hereinafter referred to as the "liquid crystal display element" or "LCD") of the active matrix liquid crystal display device, comprises a pair of substrates confronting one another through the liquid crystal layer, at least one of which is transparent. One of the substrates generally has a plurality of gate lines extended in an x-direction and juxtaposed in a y-direction, and a plurality of data lines extended in the y-direction across the gate lines through an insulating film and juxtaposed in the x-direction, formed over its surface on the side of the liquid crystal layer. A region enclosed by those individual lines forms a unit pixel region, which is equipped with a thin film transistor and a pixel electrode. The aforementioned x-direction and y-direction are in a relation to extend along the substrate surface confronting the liquid crystal layer and to intersect each other (i.e., not in parallel). In most cases, the y-direction is defined to intersect the x-direction substantially perpendicularly, but this angle of intersection could be suitably changed according to the demand from the product design so that the directions are transverse to one another. The x-direction and the y-direction to be described hereinafter will be defined, as described hereinbefore, unless otherwise specified.
The pixel electrodes are fed with video signal voltages from data lines through the thin film transistors which are turned ON when fed with the scanning signal voltages from gate lines. As a result, an electric field is established (in the case of the vertical field type) between the pixel electrodes and a common pixel electrode formed on the other opposed substrate so that the optical transmittance of the liquid crystal layer interposed between the pixel electrodes and the common pixel electrode may be modulated to effect a predetermined display.
Moreover, the gate lines, data lines, thin film transistors and pixel electrodes are formed by forming different material layers into predetermined patterns by a selective etching method using the photolithography technique and by laminating the etched material layers sequentially. Such a liquid crystal display device is disclosed in detail in Japanese Patent Application Laid-Open No. 32651/1987, for example.
In the fabrication of the aforementioned substrate (hereinafter referred to as the "TFT substrate") on the side having the gate lines, data lines, thin film transistors and so on, defects occur resulting in disconnection of the gate lines and the data lines. During the step of fabricating the TFT substrates, therefore, the disconnections are inspected by measuring the electric resistances to select the proprieties of the product. This disconnection inspecting step is performed for all the wiring lines by using inspection terminals on the TFT substrate and a plurality of inspection probes prepared according to the interval of the inspection terminals. The technique of inspecting the disconnections of the adjoining wiring lines of the gate lines or data lines by connecting them in series is disclosed in Japanese Patent Application Laid-Open No. 124825/1989 or 1825/1990.
Both Japanese Patent Application Laid-Opens Nos. 124825/1989 and 1825/1990 describe the problem of gate dielectric breakdown due the static electricity in the process after the disconnection inspections. As a solution of the problem, although different in the two disclosures, there has been disclosed a construction of two adjoining gate lines, two adjoining data lines and a film of ITO short-circuiting one end of the gate lines and one end of the data lines. As a matter of fact, however, there is another problem to be solved in addition to the aforementioned defect occurrence due to the static electricity. This serious problem is that the TFT substrate conforming to the disconnection inspection (or having no wiring defect) is misjudged as being defective.
In the disconnection inspection, when the electric contact between the inspection probe and the inspection terminal is insufficient, the line (e.g., the gate line or the data line) connected with the inspection terminal is determined to be defective, and the TFT substrate having that line is rejected. Consequently, the yield in the number of supplied TFT substrates is reduced. This defective contact occurs mainly when the inspection probe and the inspection terminal are not positioned accurately. This problem becomes serious in the case of taking a plurality of TFT substrates from a large-sized glass substrate, or in the case of a high definition liquid crystal display device having inspection terminals arranged at narrow intervals. As a size of the glass substrate becomes larger, it is more difficult to retain its positioning accuracy.
Since the inspection probe or the inspection apparatus equipped with a plurality of inspection probes is prepared in conformity to the requirement of the products, different inspection probes corresponding in number according to the number of kinds of the products have to be prepared for fabricating the products simultaneously. This is apparent in view of Japanese Patent Application Laid-Open No. 199210/1995 which has disclosed a inspection terminal structure suited for a high definition liquid crystal display device. The inspection terminals disclosed thereby, are so different from those disclosed in Japanese Patent Application Laid-Open No. 124825/1989 that the disconnection inspection of the former cannot be performed by the apparatus used in the latter disconnection inspection.